Iranian warplanes fired multiple rounds at an unarmed US surveillance aircraft in international airspace over the Gulf last week, but the craft was undamaged and returned safely to its base, Pentagon spokesman George Little said on Thursday.

US president Barack Obama and defence secretary Leon Panetta were both advised early on about the unprecedented incident, which happened about 4:50am on November 1 (local time).

Iran was later warned through diplomatic channels the United States would keep conducting surveillance flights in the region and would protect its military assets, Mr Little said.

"The United States has communicated to the Iranians that we will continue to conduct surveillance flights over international waters over the Arabian Gulf consistent with longstanding practice and our commitment to the security of the region," he told a Pentagon briefing.

"We have a wide range of options, from diplomatic to military, to protect our military assets and our forces and [we] will do so when necessary," he added.

The incident came a year after a CIA drone crashed in Iran - giving the Islamic Republic access to sensitive American technology.

The MQ1 Predator military drone involved in the latest incident was conducting routine surveillance over the Gulf, 16 nautical miles from Iran, when it was intercepted by Russian-made Iranian SU-25 Frogfoot aircraft and was fired upon with guns, Mr Little said.

Another Pentagon spokesman confirmed two SU-25s were involved.

Asked whether the Iranians may have been firing warning shots, Mr Little said the US assessment was that the Iranian aircraft were aiming to shoot down the drone.

"Our aircraft was never in Iranian air space. It was always flying in international air space. The internationally recognised territorial limit is 12 nautical miles off the coast and we never entered the 12-nautical-mile limit," he said.

The Pentagon said relevant officials in Congress were informed of the incident.

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